1. Field of the Invention
Embodiments of the present invention relate to the field of numerically-controlled oscillators. More particularly, embodiments of the present invention relate generally to minimizing jitter in a clock generated by a numerically-controlled oscillator.
2. Related Art
Numerically-controlled oscillators (NCOs) are digital circuits that are commonly used for clock frequency synthesis and control based on an accumulator and control register. An NCO's output clock average frequency can be made to be arbitrarily accurate over some duration that includes many output clock cycles. Frequency precision is determined by the input clock frequency and NCO accumulator width. In particular, NCOs are often used in digital PLL implementations and are analogous in function to voltage-controlled oscillators (VCOs) in analog PLLs. NCOs have a advantage over analog VCOs in that the frequency output can be controlled exactly and there are not the inherent issues of noise, drift, etc, that are present with VCOs.
However NCOs, being digital in nature, do suffer from jitter induced by the time discretization of clock phase. That is, any leading edge of a clock pulse of an NCO clock signal is constrained to align with the occurrence of an input clock edge. Therefore, the time from one NCO clock edge to the next can vary by one input clock period. While the average frequency of the NCO clock can be made as precise as desired, there is always jitter on the clock that is equal to the period of the input clock.